Developing practical and efficient procedures for isolating the precursor placental (trophectoderm) and fetal (inner cell mass) cellular components of the developing mouse blastocyst was the principal aim of this project. Our specific objective was to successfully reconstruct the blastocyst, a process referred to as blastocyst reconstitution, in an attempt to avoid fetal rejections associated with interspecies embryo transfer. By injecting the inner cell mass of one species (or genotype) into the trophoblast (trophectoderm vesicle) of the recipient genotype, it is sometimes possible to eliminate fetal loss associated with immunological rejections and, thus, increase the reproductive potential of a genetically valuable animal (e.g., rare animal model or endangered species). These experiments focused on specific questions that involved the development of effective methods for isolating and then recombining inner cell masses and zona pellucida-enclosed trophectoderm vesicles.